Heretofore, a crystal of a nitride of a Group-13 metal on the Periodic Table such as a gallium nitride (GaN) compound or the like has a wide band gap and has a direct interband transition, and is therefore used as a semiconductor material, and the crystal is used in various semiconductor devices such as light-emitting elements falling in a relatively short wavelength range, for example, UV, blue or the like light-emitting diodes, semiconductor lasers and others, as well as other electronic elements, semiconductor sensors, etc.
Recently, a crystal of a nitride of a Group-13 metal on the Periodic Table has become used also for power semiconductor device (power devices) in addition to use thereof for light-emitting devices. Consequently, development of a crystal of a nitride of a Group-13 metal on the Periodic Table that is resistant to high voltage and large current is being promoted.
In addition, these devices are desired to be formed of a material of the same type and are desired to be produced using a high-quality semiconductor substrate (self-sustainable substrate) with few crystal defects, and production techniques for a crystal of a nitride of a Group-13 metal on the Periodic Table that can be such a semiconductor substrate have become studied actively.
As a production method for a crystal of a nitride of a Group-13 metal on the Periodic Table, there are known a vapor-phase growth method such as a hydride vapor-phase epitaxial (HVPE) method or the like, and a liquid-phase growth method such as an ammonothermal method, etc.
The HVPE method is a method where a Ga chloride and a hydride of a Group-5 element on the Periodic Table (NH3) are introduced into a furnace in a hydrogen stream atmosphere and thermally decomposed therein, and the crystal formed through the thermal decomposition is deposited on a substrate (for example, see PTL 1).
On the other hand, the ammonothermal method is a method for producing a desired crystal material through dissolution-precipitation reaction of a feedstock, using a nitrogen-containing solvent such as ammonia or the like that is in a supercritical state and/or a subcritical state. In application to crystal growth, a supersaturated state is generated owing to the temperature difference using the temperature dependence of the feedstock solubility in the nitrogen-containing solvent such as ammonia or the like, and a crystal is precipitated out. Concretely, a feedstock crystal and a seed crystal are put into a pressure-tight chamber such as an autoclave or the like, then sealed up, and heated with a heater or the like to thereby form a high-temperature zone and a low-temperature zone inside the pressure-tight chamber, while, on one hand, the feedstock is dissolved with crystal growth on the other side to thereby produce a crystal (see PTL 2).
The ammonothermal method is more efficient in material utilization than the HVPE method, and is therefore advantageous in that the production cost can be reduced. In addition, the ammonothermal method makes it possible to increase the quality and the size of the crystal of a nitride of a Group-13 metal on the Periodic Table to be produced, and therefore, recently, practical applications of the method are being promoted. However, it is known that the crystal of a nitride of a Group-13 metal on the Periodic Table produced according to the ammonothermal method contains relatively large quantities of crystal defects and impurities, and such crystal defects and impurities are to be a factor of worsening the crystal quality. Consequently, for example, in NPL 1 and PTL 3 and 4, it has been proposed to improve the crystallinity of a crystal of a nitride of a Group-13 metal on the Periodic Table by controlling the physical properties of the crystal under specific conditions, for example, by controlling the dislocation density of the crystal and control the concentration of impurities therein.
On the other hand, a substrate of a crystal of a nitride of a Group-13 metal on the Periodic Table may be produced by processing, for example, by slicing the bulk crystal thereof having grown on the main plane of various types of crystals according to the above-mentioned method. However, according to a heretofore-known bulk crystal growing method, there occurs a problem in that the crystal defects and the warpage existing in the seed crystal are directly given to the bulk crystal as they are. As one method for solving the problem, there is known an ELO (epitaxial lateral overgrowth) method. The ELO method is a crystal growth method where a mask layer is formed on the main plane of a seed crystal and the crystal is laterally grown on the mask from the opening according to a vapor-phase growth method, and in the method, the dislocation is stopped owing to the lateral growth, and therefore, it is known that a layer with few crystal defects can be formed (for example, see PTL 5).
As still another method, for example, PTL 6 describes a technique of preventing dislocation that may propagate in the growth direction, by making a tabular crystal grow in the +c-axial direction from the side face of the seed crystal having a +C plane as the main plane, according to a vapor-phase growth method.